This invention relates generally to drive circuits for exciting the coil of the electromagnet of an electromagnetic flowmeter, and more particularly to a constant-current duty-cycle driver for this purpose which maintains current through the coil at a substantially constant level to minimize the presence of unwanted components in the signal yielded by the flowmeter electrodes.
In an electromagnetic flowmeter, the liquid whose flow rate is to be measured is conducted through a flow tube provided with a pair of diametrically-opposed electrodes, a magnetic field perpendicular to the longitudinal axis of the tube being established by an electromagnet. When the flowing liquid intersects this field, a voltage is induced therein which is transferred to the electrodes. This voltage, which is proportional to the average velocity of the liquid and hence to its average volumetric rate, is then amplified and processed to actuate a recorder or indicator.
The magnetic field may be either direct or alternating in nature, for in either event the amplitude of voltage induced in the liquid passing through the field will be a function of its flow rate. However, when operating with direct magnetic flux, the D-C signal current flowing through the liquid acts to polarize the electrodes, the magnitude of polarization being proportional to the time integral of the polarization current. With alternating magnetic flux operation, polarization is rendered negligible, for the resultant signal current is alternating and therefore its integral does not build up with time.
All of the adverse effects encountered in A-C operation of electromagnetic flowmeters can be attributed to the rate of change of the flux field (d.phi.)/dt, serving to induce unwanted signals in the pick-up loop. When the magnetic flux field is a steady state field, as, for example, with continuous d-c operation, the ideal condition d.phi./dt=0 is satisfied. But, as previously noted, d-c operation to create a steady state field is not acceptable, for galvanic potentials are produced and polarization is encountered.
In the patent to Mannherz et al., U.S. Pat. No. 3,783,687, there is disclosed an electromagnetic flowmeter in which the excitation current for the electromagnetic coil is a low-frequency wave serving to produce a periodically-reversed steady state flux field, whereby unwanted in-phase and quardrature components are minimized without giving rise to polarization and galvanic effects.
In this prior patent, the driver system for exciting the coil includes a switching means acting to periodically reverse the raw output of an unfiltered full-wave rectifier operated from an a-c power line. Because the electromagnet has a relatively high inductance, it functions as a filter choke which takes out a substantial percentage of the ripple component in the raw output of the rectifier, thereby obviating the need for filter capacitors. In this drive system, a logic circuit or divider is provided which is activated at the power line frequency (i.e., 50 or 60 Hz) to produce low frequency gating pulses for governing the electromagnetic reverse switching action.
Drive systems which are presently employed to provide excitation current for an electromagnetic flowmeter of the type disclosed in the Mannherz et al. patent utilize a constant-voltage drive. The long L/R time constant of the electromagnet produces a relatively slow magnet current rise time hence a long excitation period is required to attain a constant flux level.
Because the total voltage and R are large, to reduce the magnet time constant to usable values, a substantial amount of power has to be dissipated by the drive system. As a consequence, a great amount of energy is lost in heat and the system is inefficient in power terms.
My prior 1980 U.S. Pat. No. 4,204,240 (Schmoock), whose entire disclosure is incorporated herein by reference, discloses an energy-efficient driver for the excitation coil of an electromagnetic flowmeter in which the driver is gated at a periodic rate that is low relative to the frequency of the A-C line voltage, the driver producing a flow of current through the coil which is at a substantially constant level.
In one embodiment of the driver disclosed in the Schmoock patent, during each gating period, high voltage pulses are applied intermittently to shock excite the coil, the respective durations of the pulses being varied so as to maintain the current passing through the coil at a substantially constant level. To this end, fluctuations in the coil current are sensed to effect corrections therefor, thereby taking into account A-C line voltage variations and other variables.
Also pertinent to the present invention are the 1982 patent to Freund et al, U.S. Pat. No. 4,325,261 and the 1979 patent to Yokayama U.S. Pat. No. 4,144,751, these patents disclosing excitation coil drivers for electromagnetic flowmeters.